Scientists have drilled about two and a half kilometers beneath the seabed and found vast underground forests of "deep life," including microbes that persist for thousands, maybe millions of years, researchers said on Monday.

Feeding on nothing but the energy from rocks, and existing in a slow-motion, even zombie-like state, previously unknown forms of life are abundant beneath the earth despite extreme temperatures and pressure.

About 70 percent of the earth's bacteria and archaea - single-celled organisms with no nucleus - live underground, according to the latest findings of an international collaboration involving hundreds of experts, known as the Deep Carbon Observatory. The findings were released at the American Geophysical Union meeting in Washington.

This "deep life" amounts to between 15 and 23 billion tons of carbon, said the DCO, launched in 2009, as it nears the end of its 10-year mission to reveal the earth's inner secrets.

"The deep biosphere of the earth is massive," said Rick Colwell, who teaches astrobiology and oceanography at the Oregon State University.

He described the team's findings so far as a "very exciting, extreme ecosystem."

Among them may be the earth's hottest living creature, Geogemma barossii, a single-celled organism found in hydrothermal vents on the seafloor. 

Its microscopic cells grow and replicate at 250 degrees Fahrenheit (121 Celsius).

"There is genetic diversity of life below the surface that is at least equal to but perhaps exceeds that which is at the surface and we don't know much about it," said Colwell.

Distinct from surface life

Similar types of strange, deep life microbes might be found on the subsurface of other planets, like Mars.

"Most of deep life is very distinct from life on the surface," said Fumio Inagaki, of the Japan Agency for Marine-Earth Science and Technology.

Using the Japanese scientific vessel Chikyu, researchers have drilled far beneath the seabed and removed cores that have given scientists a detailed look at deep life.

"The microbes are just sitting there and live for very, very long periods of time," he told AFP.

Brought up from these ancient coal beds and fed glucose in the lab, researchers have seen some microbes, bacteria, and fungi slowly waking up.

"That was amazing," said Inagaki.

Scientists have found life in continental mines and boreholes more than five kilometers deep, and have not yet identified the boundary where life no longer exists, he added.